1. Field
The present invention relates generally to position determination. More specifically, the present invention relates to techniques for improving the accuracy and service availability in determining the position of a wireless terminal.
2. Background
It is often desirable, and sometimes necessary, to know the position of a wireless user. For example, the Federal Communications Commission (FCC) has adopted a report and order for enhanced 911 (E-911) wireless service that requires the location of a wireless terminal (e.g., a cellular phone) to be provided to a Public Safety Answering Point (PSAP) each time a 911 call is made from the terminal. The FCC mandate requires the location of the terminal to be accurate to certain specifications, which is within 50 meters for 67% of calls and within 150 meters for 95% of calls.
In addition to the FCC mandate, service providers have begun to recognize that location services (i.e., services that identify the position of a wireless terminal) may be used in various applications to provide value-added features that may generate additional revenue for the service providers. For example, a service provider may use location services to implement location-sensitive billing whereby different rates may be charged for calls originating from different zones. A service provider may also use location services to provide location-sensitive information such as driving directions, local information on traffic, gas stations, restaurants, hotels, and so on. Other applications that may be provided using location services include asset tracking services, asset monitoring and recovery services, fleet and resource management, personal-location services, and so on. These various applications typically require the location of each affected terminal be monitored by the system or that the terminal be able to continually update its position.
Various systems may be used to determine the position of a wireless terminal. One such system is the well-known Global Positioning System (GPS), which is a “constellation” of 24 well-spaced satellites that orbit the earth. Each GPS satellite transmits signals encoded with information that allows receivers on earth to measure the time of arrival of the received signals relative to an arbitrary point in time. This relative time-of-arrival measurement may then be converted to a “pseudo-range”. The position of a GPS receiver may be accurately estimated (to within 10 to 100 meters for most GPS receivers) based on a sufficient number of pseudo-range measurements (typically four). However, GPS signals are received at very low power levels due to the relatively large distances between the satellites and the receivers, and most GPS receivers have great difficulty receiving GPS signals inside a building, under dense foliage, in urban settings in which tall buildings block much of the sky, and so on.
In a hybrid position determination system, signals from terrestrial or earth-bound base stations in a wireless communication system may also be used in place of, or to supplement, the signals from GPS satellites to determine the position of a wireless terminal. The wireless communication system may be a cellular communication system or some other system. A “hybrid” terminal would then include a GPS receiver for receiving GPS signals from the satellites and a “terrestrial” receiver for receiving “terrestrial” signals from earth-bound base stations. Similar to the GPS signals, the time of arrival of a received terrestrial signal may be measured relative to an arbitrary point in time and converted to a pseudo-range. Pseudo-range measurements to a sufficient number of base stations (e.g., three or more) may then be used to estimate the position of the terminal. It is well known that the terrestrial pseudo-ranges based on terrestrial signals are prone to exhibit relatively large errors due to timing and hardware errors in the base stations, timing and hardware errors in the receiver, and errors due to the propagation path. Consequently, the accuracy of a position estimate derived from terrestrial pseudo-range measurements is typically worse than that derived from GPS pseudo-range measurements.
A mobile terminal may move in and out of the coverage areas of the GPS and wireless communication system. To achieve high accuracy, it is desirable to use the GPS signals as often and to the extent possible to determine a position estimate for the terminal. Moreover, to achieve higher availability and greater coverage, it is desirable to use the terrestrial signals when and to the extent needed to determine the terminal position estimate.
There is therefore a need in the art for techniques to effectively use the GPS and terrestrial signals in a manner which provides high accuracy and high availability in determining a position estimate for a wireless terminal.